The short explanation is that you basically have two obstacles to overcome when applying for a patent in order to get the application approved. The first is that you must convince the patent examiner that this invention is indeed novel and nonobvious in light of any prior art. This is the toughest and sometimes a long process that usually can take from 1 to 3 years from when the patent application is first filed.
If you get past that, the second part of getting the patent application approved is basically money. Depending on your company the total fees to get an patent application approval for the life of the patent can range from $4,870 for a small company to $9,780 for a larger company. You can learn more from this great article at https://wp.nyu.edu/dispatch/2018/11/12/inventhelp-the-vibrant-business-of-inventing/.
Most people may not be aware of this, but they do. Of the 3 different patent types, Utility and Plant patents expire after 20 years, and design patents expire after 14 years. Other reasons why a patent might come to an end is if the owner does not pay the fees through the life of the patent, if there was some type of fraud in the patent application process that was found later on, or if the inventor engaged in some legal conduct when using the patent.
Once a patent is expired for any reason the invention described in that patent becomes public domain and anyone can use it without permission. Televisions and personal computers are examples of popular inventions that are no longer covered by patents that are expired. The invention cannot be patented again, but any invention that would improve upon public domain inventions are patentable and can be submitted via a patent application. You can learn more about patenting your idea from twitter at https://twitter.com/inventhelp too.